wip: Add mix tasks for setting admin/removing staff

This adds set_admin and remove_staff mix tasks for delineating between admin and moderator.

is_admin and is_moderator are added to the user json representer for pleroma-fe to make use of.

See pleroma-fe!250 (closed), it depends on this merge request.

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The next steps would be to refactor pleroma to reference moderator as admin, as needed.